Thermionic valve circuit



Jan. 17, 1.939. J. w. WHITELEY 2,144,039

THERMIONIC VALVE CIRCUIT Filed June l5, 1955 3 Sheets-Sheet l L r 7 l `Ian. 17, 1939. J. W wHlTELl-:Y 2,144,039

THERMIONIG VALVE CIRCUIT Filed June 13, 1955 5 Sheets-Sheet 2 Jan. 17, 1939. J. W, WHITELEY 2,144,039

THERMIONIC vALvEcIRcUIT Filed June 15, 1935 5 sheets-Sheet 3 vvvvvvv Patented Jan. 17, 1939 UNITED STATES PATENT OFFICE THERMIONIC VALVE CIRCUIT Application June 13, 1935, Serial No. 26,336 In Great Britain June 15, 1934 7 Claims.

The present invention relates to thermionic valve circuits, and is more particularly concerned with thermionic valve amplifiers.

Most broadcast transmission systems at the present time suffer from the disadvantage that variations in intensity during the programme are not faithfully transmitted; passages of more than a certain loudness are attenuated in order to avoid overloading the transmitter while very soft passages are increased in loudness in order to raise them above the level of unavoidable transmission background noises.

This intensity control is generally carried out manually by the adjustment of a variable attenuator suitably arranged in the electric circuit at the transmitter, the amount of attenuation introduced being increased during loud passages and reduced during soft passages.

Similar restrictions upon the range of intensity of the matter being recorded are imposed in the manufacture of talking machine records, where considerations analogous to those discussed with reference to broadcast transmission systems apply.

It is an object of the present invention to provide an amplifier which will introduce a measure of compensation for limitations in the intensity range of signals to be amplified.

According to the present invention, a thermionic valve amplifier comprises a thermionic valve having a negative feed-back coupling between its anode and control grid circuits, means for feeding applied signals to be amplified (after amplification if desired) to said anode and grid circuit and means for varying the mutual conductance of said valve in accordance with the mean amplitude cf the applied signals, said valve being shunted across an output circuit in such a manner that the amplitude of signals set up in said output circuit is determined over a wide range of amplitude of the applied signals, by the effective impedance of said valve.

It is well known that the impedance of a thermionic valve having a negative feed-back coupling between its anode and control grid circuits is dependent upon the magnitude of the feedback coupling and upon the mutual conductance of the valve. In the arrangement set out in the preceding paragraph, the mutual conductance of the valve is varied in accordance with the intensity of the applied signals, and since negative feed-back is arranged to take place from the anode circuit to the grid circuit, the effective impedance of the valve varies widely with changes in the intensity of the applied signal. The valve may conveniently be connected in shunt with a winding of an intervalve or output transformer, or it may be connected between any two other points in the circuit such that it can serve to introduce a variable attenuation of the applied signal as the intensity of the signal varies.

When the feed-back coupling varies with frequency, the effective impedance of the valve also varies with frequency, and it can be arranged that the relative amplitudes of oscillations at various m frequencies in the output circuit vary in any desired manner in accordance with changes in the mean intensity of the applied signals. Preferably it is arranged that as the mean intensity falls, the impedance of the valve at the middle fre- 1,5 quencies falls relatively to its impedance at the extreme frequencies of the range of frequencies to be amplified.

The variable-impedance valve may be connected in series with a resistance or other impedance, so as to constitute therewith a potential divider. The signal is then applied to the ends of this potential divider, and a part of the signal including that which is set up across the valve is passed to the output circuit.

The invention will now be described by way of example with reference to the accompanying drawings, wherein Fig. l is a diagrammatic drawing to which reference will be made for purposes of explanation.

Fig. 2 shows a part of an amplifier according to the invention.

Figs. 3 and fl show modifications of the invention which are intended to operate over an extended intensity range.

Figs. 5 and 6 show practical forms of the arrangement of Fig. 4.

Fig. 'l shows a part of a further amplifier according to the invention.

Fig. 8 shows diagrammatically an amplifier in which frequency-selective expansion is introduced and Fig. 9 shows yet another amplifier according to the invention.

Like circuit elements in the several figures are given the same reference designations.

Referring to Fig. 1 a valve V having a mutual conductance g (at a particular control grid-cathode potential difference), and an anode slope resistance Ra is arranged in series with a resistance Ri, and has its anode circuit coupled to its control grid circuit by means of a negative feed-back coupling FB such that the ratio of the alternating potential set up at the grid due to the feed-back coupling to the alternating potential applied to Ra gkRa+1+R1 Thus the valve V behaves as if its impedance from anode to cathode were Ra gkRa-l-1 If the valve is of a high impedance type, such for example as a pentode, this expression is approximately equal to It is thus apparent that the effective impedance of the valve depends upon the factor k (and hence upon the nature and magnitude of the negative feed-back coupling FB), and upon the mutual conductance g of the lvalve V; it will readily be apreciated that the valve may accordingly be made to simulate any desired impedance or combination of impedances by a suitable choice of feed-back coupling, and that the effective impedance of the valve at any frequency may readily be varied by varying the mutual conductance of the valve. From Fig. 1 it will be seen that the grid and plate of tube V are connected to the same alternating potential point of R1; the internal impedance of tube V and R1 are in series across the input, and the output is across the tube impedance. As the latter varies, so will the voltage fed to the network output vary.

Referring now to Fig. 2, a thermionic amplifying valve I of the double-diode-triode type has its anode 2 connected to the positive terminal 3 of a source of anode current, indicated generically by battery B, through a coupling choke 4 in series with a decoupling resistance 5, and its cathode 6 connected to the negative terminal 'I of the source (which is preferably earthed) through a biasing resistance 6 in parallel With a by-pass condenser 9. The input terminals I!) and I I are connected to the preceding valve of the amplifier.

The anode 2 of the valve I is connected through a resistance I2 shunted by a condenser 2i to the anode of a variable-mu Valve I3, which is preferably of the pentode type, and the screening grid of this Valve is connected through a feed resistance I4 to the positive terminal 3 of the anode current source, a decoupling condenser I5 being connected between the screening grid and earth. The cathode of the Valve I3 is connected directly to the cathode of the valve I.

A part of the output of the valve I is rectied by means of the diode rectifier constituted by the diode anodes I6 and the cathode 6 of the valve I. The anodes I6 are connected to the cathode 9 through limiting resistances I'I and I8 and a load resistance I9 in series. Oscillations to be rectified are fed to the anodes I6 of the diode through a condenser 20 connected between the anode of the Valve I and the junction of the limiting resistances I 'I and I8; the function of the limiting resistance I'I is to limit the current in the diode I6, 6 and thus to limit the non-linearity of the load presented by the diode and its associated circuits to the valve I. The diode I6, 6 is shunted by a condenser 22, and a by-pass condenser 23 is connected in shunt with the resistance I9.

The end of the load resistance I9 remote from earth is connected through a resistance 24 to the control grid of the Variable-mu pentode valve I3. The grid bias, and hence the mutual conductance of the latter, is thus controlled by the rectifled potential difference set up across the load resistance I9 of the diode I 6, 6. The load resistance I9 is made as high as possible so that the rectiiier circuit presents a high impedance to the Valve I.

The anode of the pentode Valve I3 is connected to its cathode through a condenser 25 in series with the primary winding 26 of a transformer 2l; the transformer 2'I may be an intervalve coupling transformer or an output transformer; for convenience in description the secondary winding 28 of the transformer 2'! will be referred to as the output circuit of the part of the ampliiier under consideration.

It Will be appreciated that in the arrangement described, the oscillations applied to terminals II! and II are established after amplification in Valve I across a potential divider comprising the fixed resistance I2 in series with the anodecathode path of the pentode valve I3, the output being taken from across the anode-cathode path of the valve I3. The resistor I2 corresponds to resistor R1 of Fig. l, while tube I3 is the tube V of the latter. The grid and plate of tube I3 are connected to the same alternating potential point of resistor I2; the internal impedance of tube I3 shunts the output transformer 2'I. The grid bias applied between the control grid and cathode of the pentode I3 increases negatively as the amplitude of the applied oscillations to be amplied increases, owing to the rectified voltage fed from the diode I6, 6.

When the amplitude of the applied oscillations is very low, the potential on the control grid of the pentode I3 tends to become substantially equal to that of the cathode thereof, and grid current accordingly commences to flow in the resistances 29 and I9. 'I'hese resistances are given such values that at very low intensities, a small negative bias potential produced by grid current flow is maintained on the control grid of the pentode I3, which thus operates with its maximum mutual conductance.

Connected between the anode and cathode of the pentode valve I3 is a feed-back potential divider comprising a condenser 29 in series with two resistances 39 and BI, and the junction point of thesev resistances is connected to the control grid of the pentode I3 through a condenser 32. A further condenser 33 may be connected in shunt with the resistance 36, this condenser serving to compensate for the by-passing of the higher frequencies rby the primary winding 26 of the transformer 21.

As has been shown above, since the internal impedance of tube I3 is inversely dependent on the product of the magnitude of the alternating potential impressed on the grid and on the gain of the tube, the effective impedance of the variable-mu pentode I3 in the above described ampliiier varies inversely in accordance with changes in the mutual conductance of the valve, and since the output of the amplifier is taken from across the anode-cathode path of the valve I3, and since the mutual conductance thereof is made to decrease as the amplitude of the applied oscillations increases, it will. be apparent that the effective overall amplification of the amplifier increases as the amplitude of the applied oscillations increases. The amplifier network is tube I whose input terminals are I II, II; and whose output electrodes are connected to transformer 2'I. The primary 26 of the latter is in series with resistor I2; and is shunted by the internalA impedance of tube I3. In other words, I2 and I3 provide a potential divider, and tube I3 acts as a variable resistor whose magnitude increases as the input to amplifier I increases.

The time constant of the biasing circuit which controls the mutual conductance, and hence the eiective impedance of the pentode I3 is made relatively long, so that very rapid changes in level produce no effect. For example, the resistances I 'I and I8 may have the value 0.1 megohm, the resistance I9 may be 2.3 megohms, and the condenser 23 may have a capacity of the order of 0.4 microfarad; in this case, the

; time constant for the charging up of condenser 23 is about 0.1 second, and. the time constant when discharging is about 1.0 second.

Referring to Fig. 3, a thermionic amplifying valve I has input terminals I0 and Il, and has a resistance I2 in series with a variable impedance Z connected between its anode and cathode. The impedance Z may be constituted by a pentode valve of the variable-mu type, having a negative feed-back coupling between its anode circuit and itsv grid circuit, as described with reference to Fig. 2; the grid bias of this valve is then controlled in accordance with the rectified output of a rectifier 34, which is shunted by a load resistance 35.

The rectiiier 34 is fed with the amplified signal from the valve I through a variable-mu valve 36 and a coupling condenser 31, and the control grid of the valve 36 is connected to a tapping point in the resistance 35. The output terminals 38 are connected to an output circuit, if desired through a further stage of ampliiication. The operation of this arrangement is as follows:

As the intensity of the signal applied to terminals I6, I I increases, the grid bias of the valve 36 increases, and hence the gain provided by this valve decreases. Accordingly, as the signal intensity increases from a minimum value, the rate of increase of the rectified output of rectier 34, and hence of the eiiective value of impedance Z, decreases. The amount of expansion introduced thus varies more slowly than in the absence of valve 36, and it can be arranged that expansion is introduced over a wider range of intensities extending more nearly to the overload level. Expansion of amplification, or volume, is secured in the arrangements shown in this application because the ratio of ampliiier output to input increases with ampliiier input.

In the arrangement shown in Fig. 4 the rectifier 34 is connected in shunt with the resistance I2. The signal input to the rectifier is thus dependent upon the current passing through the potential divider constituted by resistance I2 and impedance Z in series, and since the effective value of impedance Z is greatest at high intensity levels, the signal input to the rectifier 34 increases more slowly as the intensity increases. The amount of expansion introduced thus varies more slowly than in the arrangement of Fig. l, for example, and expansion may be introduced over a wider range of intensity levels.

Two practical arrangements based on that of Fig. 4 are shown in Figs. 5 and 6.

Referring to Fig. 5, the amplifying valve I has input terminals I0, I-I, and has its anode connected to the positive terminal 3 of the source B of anode current through a resistance 4; the cathode 6 of valve I is connected to the negative terminal 'I of the source through a biasing resistance 8 shunted by by-pass condenser 9.

The anode of valve I is connected to the anode of a double-diode-triode valve I3, the cath-V ode of which is connected to the terminal I through resistance I2. The diode anodes I6 are connected to the cathode 43 of valve I3 through limiting resistances Il, I 8 and a load resistance I9, the latter being shunted by by-pass con denser 23. The junction point of resistances I8, I9 is connected through resistance 24 to the grid of valve I3, and a condenser 20 is connected between the junction point of resistances I'I and I 8, and the end of resistance I2 remote from the cathode 43 of valve I3.

A feed-back potentiometer 29, 30, 3l is provided as in Fig. 2, the junction point of resistances 36 and 3| being connected to the grid of valve I3 through a condenser 32.

As in the arrangement of Fig. 2, the valve I3 and resistance I2 constitute apotential divider to which oscillations from valve I are applied; the output is taken from transformer 2i, the primary winding 26 of which is connected in series with condenser 25 between the anode and cathode of valve I3. Oscillations set up across resistance I2 are rectiiied by diode I6, 43, and the grid bias of. valve I3 is controlled in accordance with these rectified oscillations. As the grid bias increases, the effective impedance of valve I3 increases, and the oscillatory potential difference across resistance I2 becomes a smaller fraction ofv the output of the valve'I. Thus as the intensity of the signal applied to input terminals I6, II increases, the rate of increase of the effective impedance of valve I3' becomes less. Volume expansion is thus introduced ove1` a wider range of intensities than is possible with the arrangement of Fig. 2.

The arrangement of Fig. 6 differs from that of Fig, 5 in that full-wave diode rectification is employed. The diodes are fed'through a transformer 44 having its primary winding 45 connected through condenser 45 in shunt with resistance I2. The transformer 44 may if desired be arranged to introduce a voltage step-up. In this case also the rate of change of the effective impedance of valve I3 falls off as the intensity increases.

In the arrangements described with reference to Figs. 2 to 6, a sudden change of the control grid bias of the valve which serves as a variable impedance (valve I3 in Figs. 2, 5 and 6) may produce a pulse of current in the output of the amplier which will give rise to a spurious signal. The generation of these spurious signals may be avoided by employing two control valves connected in push-pull, as shown in Fig. 7.

Referring to Fig. 7, an intervalve transformer 3,9 has its primary winding fed with signal oscillations from ak source such as an amplifying valve, and has two secondary windings 40 and 4I. One end of the winding 40 is connected to the anode of a variable-mu pentode valve I3 through a resistance I2, and the other end is connected to the anode of a second variable-mu pentode I3 through a resistance I2. The centre point of Winding 4I) is connected to a point 3 at a positive potential in the source B of anode current, and the screening grids of Valves I3 and I3 are connected to a point 3 in the source. The negative terminal I of the source is connected to the cathodes of the valves I3, I 3.

The ends of secondary winding 4I are connected to the two anodes of a full-Wave diode rectifier d2, the cathode of which is connected to terminal 'I'. The centre point of winding 4I is connected through limiting resistance I'I and load resistance I9 in series to the cathode of the diode 42, and resistance I9 is shunted by condenser 23. The junction of resistances I'I and 59 is connected through resistances 24 and 2d to the control grids of the valves I3 and I3 respectively. The anode of valve I 3 is connected to its cathode through condenser 29 and resistances 30 and 3| in series, and the junction of these resistances is connected to the control grid of valve I3 through condenser 32 for the purpose of providing negative feed-back.

Condensers 29', 32 and resistances 30', SI serve to provide feed-back in association with valve I3', and the anodes of the valves I3, I3 are connected together through condenser 25 and the primary winding 26 of transformer 21. The latter may be an intervalve coupling transformer, or an output transformer.

The operation of the arrangement is substantially the same as that of the arrangement described with reference to Fig. 2; as has been explained, however, since two control valves I3, i3 arranged in push-pull are employed, sudden changes in the bias on the control grids of these valves due to sudden changes in intensity do not produce spurious signals in the output of the amplifier.

The arrangements described may readily be adapted to give automatic tone control. Because of the conformation of the sensitivity characteristc of the normal human ear, it is desirable to amplify the high and low frequencies to a relatively greater extent than the middle frequencies of the audio-frequency range when the intensity level is below a certain value.

It should be noted that in cases in which a control Valve is associated with a transformer, it is undesirable that means for providing adjustable tone control should be associated with the same transformer. The reason for this is that the tone control circuits may provide a shunting impedance to the effective impedance of the control valve, and in this case the effects of variations in the effective impedance of the control Valve will differ for different settings of the tone control.

In ampliers such as those already described, if it be assumed that the level of background noise up to the point in the amplifier at which expansion is introduced remains substantially constant, then the level of background noise in the output circuit of the amplifier will fluctuate, due to the expansion introduced, in accordance with variations in intensity cf the applied signal. It is found that the rise and fall of the level of background noise is unpleasant to the listener, and means are preferably provided for obscuring fluctuations in background noise level.

For this purpose, advantage is taken of the fact that background noise is generally chiefly confined to the high frequency end of the audio frequency range; in one arrangement two control valves are provided, one being arranged to introduce expansion at the middle frequencies, and the other being arranged to operate substantially solely at the high frequencies, the latter valve being arranged to respond more rapidly to a reduction in level than the former. Fig. 9 shows a part of an amplifier employing tWo control valves.

Referring to Fig. 9, two double-diode-pentode valves I3 and I3 have their anode-cathode paths connected in parallel with one another, and in series With a condenser 25 and the primary winding 26 of a transformer 2l, which may be an intervalve transformer.

Signals to be amplified are applied to input terminals l0, lI of an amplifying valve I; the resistance 52 is connected in series with the Valves i3, I3 between the anode and cathode of valve I. The diode constituted by diode anodes I6 and the cathode of valve I3 is fed through condenser 28 and limiting resistance Il; a further limiting resistance i8 and a load resistance I9 are connected between the junction of elements Il', 25 and the cathode of valve I3, the resistance I9 being shunted by by-pass condenser 23. The junction of resistances I8 and I9 is connected through a resistance 22 to the control grid of valve I3.

The diode constituted by diode anodes I and the cathode of valve i3 is associated with a similar circuit comprising resistances Il', I 8', i9', 24 and Condensers 29 and 23 connected as shown.

Feed-back from the anode of valve I3 to the control grid thereof is effected by a potential divider comprising resistances 30 and 3l and condenser 29, the junction of resistances Sli and 3i being connected to the control grid through condenser 32. The feed-back circuit 2S, 36, 3l, 32 is arranged to introduce substantially uniform feed-back over the middle part of the audio frequency range, so that the valve I3 functions substantially as a variable resistance over this part of the operating range.

The feed-back circuit of valve I 3 comprises Condensers 29 and 32', and resistance 3V; it is arranged that this circuit introduces more feedback at high frequencies than at low frequencies, and the valve I3 thus functions as a variable capacitance, in that its impedance decreases with increase of frequency. The valve i3 accordingly serves to introduce expansion over the middle of the audio-frequency range, while the valve I3 introduces expansion only at the high frequencies.

The time constant of the circuit 23', I9 is made shorter than the time constant of circuit 23, I9 so that the valve i3' responds more rapidly to a reduction in intensity than does the valve I3. When the level of the signal falls, therefore, the background noise level falls before the general level is much reduced.

On the other hand, the time constant of circuit Il, I8, 23 is made shorter than that of circuit Il, I8', 23', so that when the general level rises, the background noise level rises only after the general level has risen considerably.

It is also found that, if means are not provided fcr obscuring fluctuations in background noise, the amplifier gives less pleasing results than when the level of background noise is substantially constant. In a further arrangement according to the invention, in order to reduce fiuctuations in background noise due to the introduction of volume range expansion, an inductance coil is arranged in series with the control valve, as Shown diagrammatically in Fig. 8, the coil serving to limit the range of control provided by the valve. In Fig. 8, the impedance Z is a valve having a negative feed-back coupling between its anode and grid circuits. The output is taken from terminals 38, and the impedance Z is connected in series with the inductance L between these terminals. For high frequencies, and when the intensity level is high, the impedance of the circuit constitued by L and Z in series issubstantially equal to Z, and the amplication at high frequencies is controlled in accordance with Z. At high frequencies and low intensity levels, the impedance of circuit LZ is substantially equal to that of L, and the amplification is substantially independent of ZL The amplification at low and middle frequencies is controlled at all levels in relation to variations in Z. The arrangement is made such that when the intensity level falls, as at the end of a musical passage, the amplification of all frequencies falls until the sound intensity reaches a medium level, after which the amplification of low and medium frequencies continues to fall, but the amplification of high frequency background noise remains substantially constant. The inverse effect isI 0btained when the intensity level rises.

Particular'attention has been paid in the above description to wireless and gramophone amplifiers in which it is required that expansion shall be introduced over substantially the whole of the range; the introduction of frequency-selective expansion has also been dealt with however, and the application of the invention to amplifiers in which expansion is desired to take place over a selected part only of the range will be clear to those versed in the art.

Many other modifications of the invention within the scope of the appended claims will readily occur to those versed in the art.

I claim:

1. A thermionic valve amplifier comprising an input circuit for receiving signals to be amplied, a thermionic valve having an anode circuit and a control grid circuit, a negative feed-back coupling for producing anti-regenerative feed-back between said anode and grid circuits, a coupling between said input circuit and said valve, means for varying the mutual conductance, and hence also the effective impedance of said valve in accordance with the mean amplitude of said signals, an output circuit and means for utilizing changes in the Value of said effective impedance to determine the amplitude of signals set up in said output circuit over a wide range of amplitude of signals in said input circuit.

2. A thermionic valve amplifier comprising an input circuit for receiving signals to be amplified, a thermionic valve having an anode circuit and a control grid circuit, a negative feed-back coupling between said anode and grid circuits for producing anti-regenerative feed-back from said anode circuitto saidgrid circuit within a part only of the frequency range of said signals, a coupling between said input circuit and said valve, means for varying the mutual conductance, and hence also the effective impedance of said valve in accordance with the mean arnplitude of said signals, an output circuit and means for utilizing changes in the value of said effective impedance to determine the amplitude of signals set up in said output circuit over a wide range of amplitude of signals in said input circuit.

3. A thermionic valve amplifier comprising an input circuit for receiving signals to be amplified, a thermionic valve having an anode circuit and a control grid circuit, a negative feed-back coupling for producing anti-regenerative feed-back between said anode and grid circuits, at least one impedance element connected in series with the anode-cathode path of said valve to constitute therewith a potential divider, means for establishing signals in said input circuit across said potential divider, means for varying the mutual conductance of said valve in accordange with the mean amplitude of said signals, an output circuit and means for feeding signals set up across a part of said potential divider including said valve to said output circuit.

4. A thermionic valve amplifier comprising an input circuit for receiving signals to be amplified, a thermionic valve having an anode circuit and a control grid circuit, a negative feed-back coupling for producing antiregenerative feedback between said anode and grid circuits, a coupling between said input circuit and said valve, a rectifier, a coupling between said input circuit and said rectifier, means for varying the grid bias, and hence the effective impedance of said valve in accordance with the rectified output of said rectifier, an output circuit and means for uitilizing changes in the value of said effective impedance to determine the amplitude of signals set up in said output circuit over a wide range of amplitude of signals in said input circuit.

5. A thermionic valve amplifier comprising an input circuit for receiving signals to be amplified, a thermionic valve having an anode circuit and a control grid circuit, a negative feed-back coupling for producing anti-regenerative feedback between said anode and grid circuits, at least one impedance element connected in series with the anode-cathode path of said valve to constitute therewith a potential divider, means for establishing signals in said input circuit across said potential divider, a rectifier, a coupling between said input circuit and said rectifier, means for varying the grid bias, and hence the effective impedance of said valve in accordance with the rectified output of said rectier, an output circuit and means for feeding signals set up across a part of said potential divider including said valve to said output circuit.

6. A thermionic valve amplifier comprising an input circuit for receiving signals to be amplified, a thermionic valve having an anode circuit and a control grid circuit, a negative feed-back coupling for producing anti-regenerative feedback between said anode and grid circuits, at least one impedance element connected in series with the anode-cathode path of said valve to constitute therewith a potential divider, means for establishing signals in said input circuit across said potential divider, a rectifier, a couplingl for establishing signals set up across a part of said potential divider across said rectifier, means for varying the grid bias, and hence the effective impedance of said valve in accordance with the rectified output of said rectifier, an output circuit and means for feeding signals set up across a part of said potential divider including saidvalve to said output circuit.

'7. A thermionic valve amplifier comprising an input circuit for receiving signals to be amplied, a thermionic valve having an anode circuit and a control grid circuit, a potential divider connected in shunt with the anode-cathode path of said valve, a connection between said grid circuit and a point in said potential divider for providing anti-regenerative feed-back, a coupling between said input circuit and said valve, means for varying the mutual conductance, and hence also the effective impedance of said valve in accordance with the means amplitude of said signals, an output circuit and means for utilizing changes in the Value of said effective impedance to determine the amplitude of signals set up in said output circuit over a Wide range of amplitude of signals in said input circuit.

JOSEPH WILLIAM WHI'I'ELEY. 

